code: def test_transformer_forward_backward(self):
      layer = ZLayer.TransformerLayer.init(
          vocab=200, hidden_size=128, n_head=4, seq_len=20
      )
  
      train_token = np.random.randint(20, size=(2, 20))
      train_pos = np.zeros((2, 20), dtype=np.int32)
      input = [train_token, train_pos]
      self.assert_forward_backward(layer, input)
  
imports: import pytest
  import numpy
  from test.zoo.pipeline.utils.test_utils import ZooTestCase
  import zoo.pipeline.api.keras.layers
  from zoo.pipeline.api.keras.models import Model, Sequential
  import keras.layers
  from keras.engine import merge, Model
  from keras.models import Sequential
  import keras.backend
  from bigdl.keras.converter import WeightsConverter
  from zoo.pipeline.api.keras import regularizers
label_desc: Use image node_id 16 for calling the TestLayer obj's underlying member method code with example usage: obj.test_transformer_forward_backward() without return types